A CD-R is generally known as a write-once type optical information recording medium which is reproduced by a compact disc (CD) drive or compact disc read only memory (CD-ROM) drive.
An optical information medium such as a CD-R has a basic structure wherein a recording layer consisting of an organic pigment film formed on a translucent substrate provided with spiral-shaped pre-grooves, a reflective layer consisting of metallic film is formed thereon, on top of which a protective layer consisting of ultraviolet-setting resin is formed, and recording is performed by irradiating a laser beam onto the recording layer from the translucent substrate side, thereby destroying the pigment in the recording layer in a localized fashion.
Due to its merits of CD compatibility and a lower cost price per bit than paper, the market for such optical information recording media is expanding rapidly, and recording devices, and media corresponding to same, which are capable of writing at recording speeds over 10 times conventional speeds, are being supplied in the market in response to demands for high-speed processing capability.
Moreover, the market is also demanding increased recording capacity in response to the year on year increases in data processing volumes, and capacity of the order of 1.3 GB (Gigabytes) is considered necessary for data volumes for image processing, and the like.
Moreover, in cases where, in order to ensure approximately double the present recording capacity without changing the external diameter of a conventional write-once type optical information recording medium, pit recording is performed using a laser beam of 780 nm wavelength, 0.45-0.50 NA (Numerical Aperture), and approximately 1.6 μm spot diameter in a current CD-ROM/-R/-RW drive, or the like, then problems of the following kind arise.
Namely, if it is attempted simply to reduce the interval of the track pitch, then during reproduction, the leakage of signals from adjacent tracks, known as “cross talk”, will increase, meaning that a clean signal cannot be obtained, and moreover, as a consequence of this, the fluctuation in the bit signal during reproduction, known as “jitter”, will also increase.
Furthermore, if it is attempted to increase the density of the recorded pits in the linear velocity direction, then a phenomenon known as thermal interference will occur, wherein the heat energy generated during pit formation will affect the recording state of the subsequent pit, and moreover, by shortening the length of the previously recorded pit and the interval from same to the next pit, the position of the subsequently written pit is more liable to be displaced, and consequently, jitter increases. This applies similarly in the case of high-speed recording.